全文获取类型
收费全文 | 458篇 |
免费 | 16篇 |
国内免费 | 1篇 |
专业分类
化学 | 282篇 |
晶体学 | 8篇 |
力学 | 21篇 |
数学 | 95篇 |
物理学 | 69篇 |
出版年
2023年 | 4篇 |
2022年 | 15篇 |
2021年 | 13篇 |
2020年 | 14篇 |
2019年 | 19篇 |
2018年 | 17篇 |
2017年 | 10篇 |
2016年 | 17篇 |
2015年 | 20篇 |
2014年 | 24篇 |
2013年 | 39篇 |
2012年 | 29篇 |
2011年 | 30篇 |
2010年 | 22篇 |
2009年 | 18篇 |
2008年 | 30篇 |
2007年 | 25篇 |
2006年 | 17篇 |
2005年 | 19篇 |
2004年 | 16篇 |
2003年 | 6篇 |
2002年 | 16篇 |
2001年 | 3篇 |
2000年 | 3篇 |
1999年 | 5篇 |
1998年 | 3篇 |
1997年 | 1篇 |
1996年 | 3篇 |
1995年 | 1篇 |
1994年 | 4篇 |
1993年 | 2篇 |
1992年 | 1篇 |
1991年 | 1篇 |
1985年 | 4篇 |
1983年 | 2篇 |
1982年 | 2篇 |
1981年 | 1篇 |
1980年 | 1篇 |
1979年 | 4篇 |
1978年 | 1篇 |
1976年 | 1篇 |
1975年 | 3篇 |
1974年 | 1篇 |
1973年 | 4篇 |
1970年 | 1篇 |
1966年 | 1篇 |
1957年 | 1篇 |
1955年 | 1篇 |
排序方式: 共有475条查询结果,搜索用时 15 毫秒
461.
The flowers of Pulicaria laciniata (Coss. et Kral.) Thell. (Asteraceae) afforded a new sesquiterpene acid 1, named lacitemzine together with the three known compounds, 4-hydroxy-3-methoxypyridine 2, β-sitosterol-3-O-β-d-glucoside 3 and 1,3,5-trimethoxybenzene 4. The structure of compound 1, 2-(2,6-dimethyl-3,4,5,6,7,8,9,10-octahydro-5,8-oxaazulen-9-yl)acrylic acid, contains a guaiane skeleton and was elucidated by spectroscopic procedures including 2D-NMR and X-ray diffraction. 相似文献
462.
In this paper we consider the Dunkl operators T
j
, j = 1, . . . , d, on and the harmonic analysis associated with these operators. We define a continuous Dunkl Gabor transform, involving the Dunkl
translation operator, by proceeding as mentioned in [20] by C.Wojciech and G. Gigante. We prove a Plancherel formula, an inversion formula and a weak uncertainty principle for it. Then, we show that the portion of the continuous Dunkl Gabor transform
lying outside some set of finite measure cannot be arbitrarily too small. Similarly, using the basic theory for the Dunkl
continuous wavelet transform introduced by K. Trimèche in [18], an analogous of this result for the Dunkl continuous wavelet
transform is given. Finally, an analogous of Heisenberg’s inequality for a continuous Dunkl Gabor transform (resp. Dunkl continuous
wavelet transform) is proved.
相似文献
463.
Synthesis and characterization of (Mo72Fe30)-coated large hexagonal gibbsite gamma-Al(OH)3 platelets
Roy S Meeldijk HJ Petukhov AV Versluijs M Soulimani F 《Dalton transactions (Cambridge, England : 2003)》2008,(21):2861-2865
The polyoxometalates or POMs (clusters comprising at least two metal and many oxygen atoms) have recently gained significant interest owing to their versatile architecture and especially their catalytic activities. Due to their high catalytic activity but low surface area, there is always a demand for making high surface area POMs. This work demonstrates the attachment of the anionic (Mo72Fe30) POMs to gibbsite nanoplatelets with a residual positive charge to form large surface area composites. The resulting composite reported here has been characterized using cryo-TEM imaging, EDX/STEM (elemental) analysis, ATR-IR spectroscopy, SAXS, electrophoretic mobility determination and XRD. The composite reported here could find application in catalysis. 相似文献
464.
Two new phytochemical compounds, Tetranins A and B, 1-(3,5'-dihydroxy-4'-méthoxyphenyl)-2-phenylethanol 1 and 5,2'-dihydroxy-5'-methoxy-6,7-methylenedioxy-isoflavone 2, were isolated from the EtOAc extract of Salsola tetrandra roots. They exhibited a significant antioxidant effect in 1,1-diphenyl-2-picrylhydrazyl (DPPH) free radical and 2,2'-azinobis (3-ethylbenzothiazoline-6-sulphonic acid (ABTS) assays. Their structures were elucidated by extensive spectroscopic methods including 1-D-((1)H and (13)C) and 2-D-NMR experiments (CHcorr, HMBC, (1)H-(1)H COSY and NOESY) as well as high-resolution ES-MS and they were found to be quite active as antioxidants in the DPPH and ABTS assays. 相似文献
465.
Ibrahim IA Qader SW Abdulla MA Nimir AR Abdelwahab SI Al-Bayaty FH 《Molecules (Basel, Switzerland)》2012,17(3):2796-2811
Current anti-gastric ulcer agents have side effects, despite the progression and expansion of advances in treatment. This study aimed to investigate the gastroprotective mechanisms of Pithecellobium jiringa ethanol extract against ethanol-induced gastric mucosal ulcers in rats. For this purpose, Sprague Dawley rats were randomly divided into five groups: Group 1 (normal control) rats were orally administered with vehicle (carboxymethyl cellulose), Group 2 (ulcer control) rats were also orally administered with vehicle. Group 3 (positive control) rats were orally administered with 20 mg/kg omeprazole, Groups 4 and 5 (experimental groups) received ethanol extract of Pithecellobium jiringa ethanol extract at a concentration of 250 and 500 mg/kg, respectively. Sixty minutes later, vehicle was given orally to the normal control group, and absolute ethanol was given orally to the ulcer control, positive control and experimental groups to generate gastric mucosal injury. The rats were sacrificed an hour later. The effect of oral administration of plant extract on ethanol-induced gastric mucosal injury was studied grossly and histology. The level of lipid peroxidation (malondialdehyde-MDA), superoxide dismutase (SOD) and gastric wall mucus were measured from gastric mucosal homogenate. The ulcer control group exhibited severe gastric mucosal injury, and this finding was also confirmed by histology of gastric mucosa which showed severe damage to the gastric mucosa with edema and leucocyte infiltration of the submucosal layer. Pre-treatment with plant extract significantly reduced the formation of ethanol-induced gastric lesions, and gastric wall mucus was significantly preserved. The study also indicated a significant increase in SOD activity in gastric mucosal homogenate, whereas a significant decrease in MDA was observed. Acute toxicity tests did not show any signs of toxicity and mortality up to 5 g/kg. The ulcer protective effect of this plant may possibly be due to its preservation of gastric wall mucus along with increased SOD activity and reduction of oxidative stress (MDA). The extract is non-toxic, even at relatively high concentrations. 相似文献
466.
467.
A screen-printed carbon electrode modified with both HRP and LOD (SPCE–HRP/LOD) has been developed for the determination of l-lactate concentration in real samples. The resulting SPCE–HRP/LOD was prepared in a one-step procedure, and was then optimised as an amperometric biosensor operating at [0, −100] mV versus Ag/AgCl for l-lactate determination in flow injection mode. A significant improvement in the reproducibility (coefficient variation of about 10%) of the preparation of the biosensors was obtained when graphite powder was modified with LOD in the presence of HRP previously oxidised by periodate ion (IO4−). Optimisation studies were performed by examining the effects of LOD loading, periodation step and rate of the binder on analytical performances of SPCE–HRP/LOD. The sensitivity of the optimised SPCE–HRP/LOD to l-lactate was 0.84 nA L μmol−1 in a detection range between 10 and 180 μMol. The possibility of using the developed biosensor to determine l-lactate concentrations in various dairy products was also evaluated. 相似文献
468.
Hisham K. Fouad 《Journal of Radioanalytical and Nuclear Chemistry》2010,285(2):193-197
The tetravalent and hexavalent uranium content of three Egyptian phosphate type ore samples namely; Sebayia, Abu Tartur and
Qatrani have been studied through selective leaching by hydrochloric acid at normal, oxidized and reduced conditions at an
amount of hydrochloric acid less than the stoichiometric value i.e. before phosphoric acid production. Oxidizing condition
is attained by incorporating 2% of manganese dioxide in the leaching cycle, whereas reducing condition is attained by adding
2% iron powder. The achieved results show that the amount of tetravalent uranium varies between 5 and 95%. As soon as the
achieved stoichiometric value of hydrochloric acid is sufficient to produce phosphoric acid both tetravalent and hexavalent
uranium dissolve by virtue of phosphoric acid complexing power for uranium. The chemical form of uranium in the ore determines
the type of solvent needed to recover it. 相似文献
469.
Hisham K. Fouad Abram F. Bishay 《Journal of Radioanalytical and Nuclear Chemistry》2010,283(3):765-772
Uranium uptake from acidic solutions is comprised practically in this study into three main steps namely; adsorbent synthesis,
uranium uptake procedure, and desorption step. In this respect, two uranium adsorbents were synthesized from mineral processing
of ilmenite and talc. Titanium phosphate adsorbent (TP) was deposited from titanium sulfate solution obtained from ilmenite
sands processing. On the other hand, magnesium silicate adsorbent (MS) is prepared by sodium metasilicate neutralization of
the acidic magnesium bearing waste solution resulted from talc whitening process. Structurally and chemically the two adsorbents
were investigated by XRD, IR and SEM-EDX analyses. The studied factors affecting the uranium uptake onto TP and MS adsorbents
were uranium concentration (10–1000 ppm), acidic pH range (1–6), contact time, shaking time and solid to liquid ratio. The
uranium analysis was determined spectrophotometrically using arsenazo(III) dye where SEM-EDX analysis confirmed the uranium
uptake by adsorbents. The optimum conditions obtained were applied to uranium bearing, highly mineralized granite samples
(5200 ppm U) and black shale (40 ppm U). The uranium uptake was more than 98% for the mineralized granite samples and more
than 97% for shale. The loaded uranium was recycled by using 0.5 and 1M HNO3 in case of TP and MS with percentage recovery of 96 and 98% respectively. 相似文献
470.
Hatem M. Titi Israel Goldberg 《Acta Crystallographica. Section C, Structural Chemistry》2009,65(12):o639-o644
The structure of 1‐benzofuran‐2,3‐dicarboxylic acid (BFDC), C10H6O5, (I), exhibits an intramolecular hydrogen bond between one –COOH group and the other, while the second carboxyl function is involved in intermolecular hydrogen bonding to neighbouring species. The latter results in the formation of flat one‐dimensional hydrogen‐bonded chains in the crystal structure, which are π–π stacked along the normal to the plane of the molecular framework, forming a layered structure. 1:1 Cocrystallization of BFDC with pyridine, phenazine and 1,4‐phenylenediamine is associated with H‐atom transfer from BFDC to the base and charge‐assisted hydrogen bonding between the BFDC− monoanion and the corresponding ammonium species, while preserving, in all cases, the intramolecular hydrogen bond between the carboxyl and carboxylate functions. The pyridinium 2‐carboxylato‐1‐benzofuran‐3‐carboxylic acid, C5H6N+·C10H5O5−, (II), and phenazinium 3‐carboxylato‐1‐benzofuran‐2‐carboxylic acid, C12H9N2+·C10H5O5−, (III), adducts form discrete hydrogen‐bonded ion‐pair entities. In the corresponding crystal structures, the two components are arranged in either segregated or mixed π–π stacks, respectively. On the other hand, the structure of 4‐aminoanilinium 2‐carboxylato‐1‐benzofuran‐3‐carboxylic acid, C6H9N2+·C10H5O5−, (IV), exhibits an intermolecular hydrogen‐bonding network with three‐dimensional connectivity. Moreover, this fourth structure exhibits induction of supramolecular chirality by the extended hydrogen bonding, leading to a helical arrangement of the interacting moieties around 21 screw axes. The significance of this study is that it presents the first crystallographic characterization of pure BFDC, and manifestation of its cocrystallization with a variety of weakly basic amine molecules. It confirms the tendency of BFDC to preserve its intramolecular hydrogen bond and to prefer a monoanionic form in supramolecular association with other components. The aromaticity of the flat benzofuran residue plays an important role in directing either homo‐ or heteromolecular π–π stacking in the first three structures, while the occurrence of a chiral architecture directed by multiple hydrogen bonding is the dominant feature in the fourth. 相似文献